1. Field of the Invention
The present invention relates to a process for producing galvanized non-aging steel sheets having good formability using low-carbon Al-killed steels with high production efficiency in a continuous galvanizing line of in-line annealing type.
2. Description of the Related Art
In recent years the tendencies in this field are toward the use of increasing amount of surface treated steel sheets for the purpose of improving the anti-rust property of steel sheets used in automobiles. Among the surface treated steel sheets, galvanized cold rolled steel sheets have been most commonly and widely used, and they are generally classified into two types: "as galvanized" and "galvannealed" (galvanized and alloyed). The galvanized and galvannealed sheets show remarkably improved spot-weldability as well as improved paint adhesion and corrosion resistance after paint coating due to the formation of the Fe-Zn alloy layer in the Zn surface layer.
The galvanized cold rolled steel sheets to which the present invention relates include from soft-grade cold rolled steel sheets having a tensile strength of 30 Kgf/mm.sup.2 order to high strength grade cold rolled sheets having 35 to 45 Kgf/mm.sup.2 order. The high strength grade sheets are particularly important because they can contribute for the weight reduction of automobiles which in turn contributes to improve the fuel consumption rate. This has been of increasing concern from the view point of the environment protection of the earth.
The conventional in-line annealing type continuous production of galvanized steel sheets with a high production efficiency generally comprises the following steps. First prior to the galvanizing, the steel strip is heated in a reducing atmosphere. This heating serves not only to clean the strip surface, but also to promote the recrystallization of the steel strip simultaneously. Thereafter, the steel strip is cooled, immersed in the zinc bath, and if the case needs, subjected to an alloying treatment, to obtain final galvanized sheet products. As understood from the above general description of the in-line annealing type production, it is a very rationalized and economical continuous production line.
Meanwhile the galvanized cold rolled steel sheets must have excellent formabilities and must be non-strain-aging, which are required by their final uses. The strain-aging is caused by carbon and nitrogen remaining in solid solution in the steel sheets and develops as surface defects called "stretcher strain" after press formings, or in the mono-axial tensile tests, it appears as material deteriorations along the lapse of time such as the increase of yield strength (YP), the lowering of elongation (El) and yield point elongation (YP-El).
Conventionally the galvanized cold rolled steel sheets satisfying the above requirements of the material qualities have been produced mainly by the following two production methods.
The first method uses a super low carbon steel containing carbides and nitrides forming elements, such as Ti and Nb, and this method enables the production of galvanized steel sheets having excellent formability and free from the strain-aging in the in-line annealing type continuous galvanizing line. However as this method requires the addition of highly costing Ti and Nb and a vacuum degassing treatment of molten steel, the method is disadvantageous in that the material cost remarkably increases. Further, regarding the material qualities, this method has the following disadvantages.
(1) As carbon and nitrogen are almost completely fixed in the steel sheets obtained by this method, no satisfactory bake hardenability (hereinafter abridged as BH) is achieved, although the non-strain-aging property is satisfied, so that the resistance to dent is poor.
(2) As carbon and nitrogen are almost completely fixed as mentioned above, carbon and nitrogen are no longer present in the grain boundaries so that during the alloying treatment, in particular, Zn will intrude into the grain boundaries. This Zn will cause surface defects such as outbursts, and deterioration of the formability due to the grain boundary embrittlement.
(3) During the reduction step, the surface of the sheets being treated will be excessively activated, causing the formation of brittle gamma phase in the intersurface between the steel substrate and the zinc coating, which in turn causes a poor adhesion of the zinc coating or requires modification or adjustment of the Al concentration in the zinc bath.
Meanwhile the second method uses low-costing low carbon Al-killed steels as the starting material. However, in the conventional continuous galvanizing line, the sheets from this material contain a large amount of carbon remaining in solid solution which will cause remarkable strain aging of the sheets. This is particularly remarkable in the case of low carbon Al-killed steels containing positively added phosphorus. Therefore this method requires a batch type post-annealing step as a necessity in order to reduce the amount of carbon in solid solution, which inevitably results in an unduly elongated production process, thus failing to take full advantage of the highly efficient continuous galvanizing production line. Further, after the post-annealing, the amount of carbon in solid solution is excessively reduced so that the desired BH property disappears.
The present invention has been completed to solve the above mentioned problems of the conventional production methods for galvanized steel sheets, and the features of the present invention reside (1) in the use of low costing, low carbon Al-killed steel as the starting material, and (2) the adoption of a heat cycle in the continuous production line of galvanized steel sheets, which heat cycle has been established on the basis of the kinetic theories of the nucleation and growth of cementite.
Over-aging treatments of continuous galvanized steel sheets have conventionally been performed in the production line by various methods as disclosed in Japanese patent Publications Sho 56-11309, Sho 60-8289, Sho 63-52088, Japanese Laid-Open Patent Applications Sho 56-51531, and Sho 60-251226.
The method disclosed in Japanese Patent Publication Sho 56-11309 comprises immersing a cold rolled sheet from a temperature of not lower than 550.degree. C. directly into a molten zinc bath controlled at about 460.degree. C. to galvanize the sheet and simultaneously to dissolve the carbon in the sheet oversaturately in solid solution by the rapid cooling achieved by the immersion, then subjecting the galvanized sheet to an over-aging treatment in a temperature range from 300.degree. to 460.degree. C. to improve the formability of the sheet. This method, however, so far as the present inventors carefully studied and found, has the following defects.
(1) The direct immersion of the sheet into the molten zinc bath from a high temperature impairs the adhesion of the zinc coating.
(2) With the quenching in the zinc bath at about 460.degree. C. and the subsequent over-aging in the temperature range from 300.degree. to 460.degree. C., the amount of carbon in solid solution will not be reduced (for example lower than 6 ppm) enough to achieve the non-strain-aging property, except when the sheet is subjected to a long time overaging treatment at a low temperature as 300.degree. C.
(3) If the overaging temperature exceeds 370.degree. C., the zinc deposited on the sheets adheres to the hearth rolls during the overaging treatment, causing surface defects on the galvanized sheets.
The methods disclosed in Japanese Patent Publications Sho 60-8289 and Sho 63-52088 have the same basic technical concept in the following points. Thus the sheets galvanized in a continuous galvanizing line are forcedly cooled and continuously overaged in the same production line. For the overaging, the galvanized sheets are rapidly heated to the overaging temperatures. According to Japanese Patent Publication Sho 60-8289, the overaging is performed in the range from 300.degree. to 600.degree. C., and according to Japanese Patent Publication Sho 63-52088, the overaging is performed in the range from 340.degree. to 370.degree. C. when no subsequent alloying treatment is to be performed, and in the range from 425.degree. to 460.degree. C. when the subsequent alloying treatment is to be done, and then the sheets thus overaged are slowly cooled.
So far as the present inventors have studied the above two prior art methods, they have the following technical problems.
(1) When the overaging treatment of the galvanized sheets is performed at a temperature exceeding 370.degree. C., the zinc deposited on the sheets adheres to the hearth rolls, causing surface defects on the sheets.
(2) According to Japanese Patent Publication Sho 60-8289, the sheets are rapidly heated with a heating rate of 50.degree. C./s or higher to the overaging temperature so as to induce dislocations in the steel matrix and to precipitate the carbon in solid solution thereinto. However the careful studies by the present inventors revealed that the precipitation site of the carbon in solid solution is predominated by MnS already existing in the grains, and the rapid heating is not always necessary.
(3) The precipitation rate of carbon during the overaging depends on the degree of oversaturation of carbon before the overaging treatment. However, this prior art publication provides no sufficient disclosure in this regard, and so far as understood, the oversaturation degree can never be satisfactory.
(4) The amount of carbon in solid solution remaining after the overaging cannot be reduced enough (6 ppm or less) to assure the non-strain-aging property by the prior art of Japanese patent Publication Sho 63-52088 because of the high overaging temperature.
According to Japanese Laid-Open patent Application Sho 56-51531, the steel sheets are subjected to a recrystallization annealing, rapidly cooled to a temperature ranging from 300.degree. to 500.degree. C. at a cooling rate of 70.degree. C./s or higher, then held in the same temperature range for 10 seconds or longer to perform the overaging. The galvanizing is performed before or after the overaging treatment.
The studies by the present inventors on this prior art revealed the following technical problems.
(1) For the nucleation of cementite in the grains, the holding of the sheets at the final point of the rapid cooling temperature range in this prior art is effective, but the holding time is too long. It has been found by the present inventors that the carbon can diffuse during the reheating to the galvanizing temperature and can form enough nuclei of cementite in the grains and that a holding time less than 10 seconds is enough or even no holding is necessary.
(2) The holding for 10 seconds or longer is too long for industrial applications to practical continuous galvanizing lines, and inevitably requires an increased size of plants and equipments.
(3) As the cementite nuclei formed in the grains by the holding at the final temperature of the rapid cooling are dissolved and disappear during the reheating, the prior art is limited to the galvanizing process and does not suggest the galvannealing process which is performed at temperatures higher than the galvanizing temperatures.
(4) For the purpose of preventing the surface defects caused by the zinc adhesion on the hearth rolls, it is necessary that the sheet temperature at which the sheet contacts the hearth rolls for the first time after the galvanizing treatment is not higher than 370.degree. C. Therefore the prior art is susceptible to this type of surface defects.
Further, the production of galvanized high-strength cold rolled steel sheets has been done in the continuous galvanizing line using a low carbon Al-killed steel with positive addition of phosphorus as disclosed in Japanese Patent Publication Sho 56-14130. However, this prior art teaches nothing of the overaging treatment and the galvanized sheets obtained by this prior art are supposed to be very inferior with respect to the non-strain-aging property. Also Japanese Laid-open patent Application Sho 62-4860 teaches a similar method, but is basically different from the present invention, and the desired non-strain-aging property can never be obtained by the overaging treatment disclosed by the prior art publication. Still further, Japanese Laid-Open Patent Application Sho 60-190525 discloses a heat cycle for a non-aging property similar to the present invention. However this prior art publication discloses nothing of the galvanizing process or the alloying process.
The galvanized steel sheets, in general, show inferior formability as compared with their substrate steel because of the presence of the zinc layer or the zinc-iron alloy layer on the surface. Therefore, it is very important for assuring excellent formability of the galvanized sheets that the formabilities of the substrates are improved beforehand. For assuring good formability of low carbon Al-killed steel sheets, the following basic considerations are essential.
(1) The cementite in the hot rolled sheet should be coagulated and coarsened, and (2) the precipitation of AlN should be fully promoted to coarsen the grains.
For these purposes, the high temperature coiling of hot rolled strips have been conventionally adopted.
However, the high temperature coiling technics are accompanied by the following two technical problems.
(1) Both leading and tailing ends of the hot strip are subjected to a rapid cooling and deteriorated in material qualities. Therefore these end portions must be cut off, causing a lowered production yield.
(2) The scale on the hot rolled strip is increased by the high temperature coiling, causing difficulties in the acid pickling and hence a lowered production efficiency.
For solving the above technical problems of the high temperature coiling, the low temperature coiling technics have been proposed. However, the low temperature coiling is effective only to improve the production yield and efficiency. Meanwhile from the point of improving the formability, in the present invention the coiling temperature may be lower or higher so far as the formability is improved.